The United Nations warned two weeks ago the armed group appeared to be have killed dozens of civilians since Iraqi forces launched the offensive against Mosul.

Aid organizations, local officials, and Mosul residents have cited reports that ISIL executed people in Hammam al-Alil and barracks nearby over the course of a week, on suspicion of planning rebellions in-and-around Mosul to aid the advancing troops.

Abdul Rahman al-Waggaa, a member of the Nineveh provincial council, told Reuters news agency last month most of the victims were former police and army members.

ISIL had used the town's agricultural college as "a killing field" for hundreds of people in the days before the Iraqi government advance, Ahmed said.

"They would torture them inside and then take them out of the neighbourhood and either shoot them or slit their throats."

Police backed up his accounts, but the road to the college was still lined with improvised explosive devices (IEDs) on Monday, preventing Reuters from visiting.